(i) Field of the Invention
The present invention relates to a multi-layer film having an oxygen absorption ability and a method for preparing the same. More specifically, it relates to an oxygen-absorbing multi-layer film which comprises a deoxidizing resin layer as a middle layer comprising a deoxidizing resin composition in which a particulate oxygen absorbing composition is dispersed, an oxygen-permeating resin layer comprising a thermoplastic resin laminated on one surface of the middle layer, and a gas barrier layer laminated on the other surface of the middle layer via a smoothing layer comprising a polyolefin resin which can compensate roughness or unevenness occasionally generated by the particulate oxygen absorbing composition to impart a smooth surface onto the smoothing layer; and a method for preparing the oxygen-absorbing multi-layer film.
The oxygen-absorbing multi-layer film of the present invention is a highly practical oxygen-absorbing packaging material which is free from roughness on the surface of the film and which is excellent in surface smoothness and heat-sealing properties.
(ii) Description of the Related Art
Heretofore, a technique is known in which a multi-layer packaging material for forming packaging bags and packaging containers can be prepared by using, as a deoxidizing resin layer, a film or a sheet obtained by molding a thermoplastic resin in which a deoxidizing agent is dispersed, whereby an oxygen-absorbing ability is held in the packaging material itself. As the deoxidizing agent which can be used in the deoxidizing resin layer of the packaging material having the oxygen-absorbing ability, there has often been used, for example, a deoxidizing agent containing, as an essential component, an iron powder which is excellent in the oxygen-absorbing ability, as suggested by Japanese Patent Publication Nos. 32348/1986 and 1824/1987, Japanese Patent Application Laid-open No. 90847/1992, and the like. Furthermore, in these publications, an oxygen absorbing multi-layer constitution is disclosed in which outside the deoxidizing resin layer in which the deoxidizing agent is dispersed, a gas barrier layer for blocking the permeation of oxygen is laminated, and inside the deoxidizing resin layer, an isolating layer having an oxygen permeability is laminated which prevents the deoxidizing resin layer from directly coming in contact with a packaged matter and which is present on the side of the packaged matter.
As a method for preparing such an oxygen absorbing multi-layer material, there can be employed various lamination methods in compliance with materials and constitutions of layers. According to a conventional technique, particularly as the deoxidizing resin layer, there has been used a resin composition obtained by blending a polyolefin resin with an iron powder type deoxidizing agent, and as the isolating layer, there has often been selected a polyolefin resin which is compatible with the deoxidizing resin layer and which has an oxygen permeability. These layers may simultaneously be molten and extruded to laminate them, or alternatively, a film which will be the deoxidizing resin layer may be stuck on a film which will be the isolating layer.
However, when it is attempted to prepare the relatively thin oxygen absorbing multi-layer film containing the deoxidizing resin layer using the resin composition obtained by blending the particulate oxygen absorbing composition with the thermoplastic resin in accordance with the above-mentioned conventional technique, the particles of the particulate oxygen absorbing composition in the deoxidizing resin layer protrude on the side of the isolating layer, so that the film having the isolating layer of the smooth outer surface cannot be obtained. In an extreme case, the particles are exposed through the outer surface of the isolating layer, with the result that the deoxidizing resin layer might inconveniently directly come in contact with the packaged matter to contaminate it. Furthermore, when it is attempted to form the gas barrier layer, particularly a resin layer having gas barrier properties and a low melting point on the other surface of the deoxidizing resin layer by hot lamination, there occurs a problem that the smoothness of the outer surface of the gas barrier layer is lost as described above, and on occasion, the particles of the deoxidizing agent is exposed through the outer surface of the gas barrier layer, so that the gas barrier properties are impaired. In such a case, an satisfactory multi-layer film cannot be obtained.
Such a problem scarcely occurs in a thick sheet-like oxygen absorbing multi-layer material, and only in the case of the thin multi-layer film, the problem is caused owing to the particulate oxygen absorbing composition of the deoxidizing resin layer. That is to say, when the particulate oxygen absorbing composition to be used is smaller in particle diameter than the thickness of the oxygen absorbing multi-layer material, the roughness in the deoxidizing resin layer is small, and this roughness can be compensated by another layer. However, with regard to the film having a thickness of about 200 .mu.m or less, the particle diameter of the particulate oxygen absorbing composition must be extremely reduced. However, it is very difficult to prepare the particulate oxygen absorbing composition of the fine particles, and the cost of the particulate oxygen absorbing composition thus prepared inevitably increases. In addition, the formation of the fine particles naturally leads to the increase in surface area, so that the obtained fine particles react with oxygen during their handling, which causes the danger of ignition.
In the present circumstances, therefore, the practical thin oxygen absorbing multi-layer material cannot be manufactured by the conventional methods.